1. Field of the Invention
The present invention relates generally to microwave energy transfer apparatus and in particular to an improved transition structure for transferring microwave energy from a strip-type transmission line to a loaded waveguide.
2. Decription of the Prior Art
Microstrip transmission lines comprising flat, narrow conductors etched or otherwise suitably fixed on a printed circuit board (which provides a spaced ground plane) have come into common use at radio frequencies. These transmission lines are particularly advantageous because of their light weight, small volume, and ease of printing. Furthermore, microstrip circuitry is easily coupled to electrical components and has wide band, low loss characteristics.
Generally, systems involving the combination of waveguide and microstrip are uncommon due to the physical size of the waveguide that is required to transmit the frequency bands normally used in microstrip lines.
Where both waveguide and microstrip lines must be used in the same system a probe structure is usually employed which effects a conversion of the microwave energy from its waveguide mode to a coaxial mode and then to a microstrip mode. Usually such probe transition devices are placed at right angles to the waveguide and protrude up through a wall into the waveguide.
There are, however, system applications where a simple, direct transition from microstrip to waveguide is desirable. For example, most ferrite phase-shifters made for microstrip are sensitive to dimensional characteristics and thus are extremely frequency-sensitive devices. In contra distinction, waveguide, ferrite phase-shifters have inherent, low-loss properties thus making them almost frequency independent. Clearly a small, direct-transition device that could be used to convert back and forth between microstrip and waveguide lines is highly desirable. For then, any frequency-sensitive microstrip device could be easily replaced by its frequency-independent, waveguide counterpart.